Friday, June 12, 2009

All resources are relative to the namespace in WPF

In Windows Presentation Foundation all resources (like ImageSources or Resource packs) are relative to the namespace of the control. So if you have a folder Images that holds your application icons and you use (in the application XAML) an ImageSource of "Images/MyImage.png" it will work. But as soon as the XAML is moved somewhere in a UserControl in the Controls namespace of a library, the ImageSource becomes invalid. It would only work as "../Images/MyImage.png" or "/Images/MyImage.png", even if the Controls namespace is not a folder and the control is in a library, not in the application that runs it!

This is part of a larger idea, that is Pack URIs. Pack URIs are used by the Open Packaging Conventions and are of the format pack://authority/path.

Long story short, authority can be application:/// or siteoforigin:///. The slashes at the end get escaped as commas in a pack URI so you get to have stuff like pack://application:,,,/ResourceFile.xaml. The path is more tricky: AssemblyShortName[;Version][;PublicKey];component/Path.
Attention at the component keyword. It is NOT a folder, but a keyword that must be there when referencing resources from an external library.

Example: I have the Resources/Images/image.png in a library and I want to use it in the XAML that is in the same library. The image source would be something like pack://application:,,,/libraryNameSpace;component/Resources/Images/image.png. Again look out for the component keyword.